Inspection of optical phase-shifting masks with an automated electron-beam system

Abstract

There has been great interest lately in using optical masks that contain phase-shifting structures in order to print smaller features than is possible with a standard binary mask. Because of the many varieties under investigation and the complexity of the phase interactions, no definitive experimental work concerning the effects of various types of defects yet exists. However, it is known that, under some circumstances, defects in a phase-shifting mask (PSM) will cause a larger deviation in the printed linewidth than a defect of equal size on a binary mask. For 0.25 μm lithography done with a 248 nm stepper using an ‘‘etched Levinson’’ or alternating PSM, it is thought that phase defects as small as 80 nm need to be detected. At present, the only inspection system capable of meeting the size requirements is based on electron optics (KLA SEMSpec). However, this type of system, which was designed to inspect x-ray masks, requires that the defects exhibit either a topographical or material anomaly in order to be visible and that the mask be conductive. While some types of phase-shifting masks satisfy the contrast formation requirements, none are conductive. This can be remedied by depositing a metallic layer over the surface. However, in order for inspection to proceed at high speed, the defect contrast and signal level must be high. In this article we elucidate the physics of electron-beam image formation of conductively coated phase-shifting masks for various types and thicknesses of coatings. Both theoretical and experimental results are presented. Since adding and removing the conductive coating must be done without damaging the mask, only certain types of coatings and processes are allowable. A list of the most promising coatings is presented along with the supporting experimental evidence. By inspecting a phase-shifting mask that has intentionally written defects of various types, it is shown that defects as small as 0.1 μm can be reliably found.